IteratedDominanceFrontier.h   [plain text]

//===- IteratedDominanceFrontier.h - Calculate IDF --------------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
/// \brief Compute iterated dominance frontiers using a linear time algorithm.
///
/// The algorithm used here is based on:
///
///   Sreedhar and Gao. A linear time algorithm for placing phi-nodes.
///   In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of
///   Programming Languages
///   POPL '95. ACM, New York, NY, 62-73.
///
/// It has been modified to not explicitly use the DJ graph data structure and
/// to directly compute pruned SSA using per-variable liveness information.
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ANALYSIS_IDF_H
#define LLVM_ANALYSIS_IDF_H

#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"

namespace llvm {

class BasicBlock;
template <class T> class DomTreeNodeBase;
typedef DomTreeNodeBase<BasicBlock> DomTreeNode;
template <class T> class DominatorTreeBase;

/// \brief Determine the iterated dominance frontier, given a set of defining
/// blocks, and optionally, a set of live-in blocks.
///
/// In turn, the results can be used to place phi nodes.
///
/// This algorithm is a linear time computation of Iterated Dominance Frontiers,
/// pruned using the live-in set.
/// By default, liveness is not used to prune the IDF computation.
class IDFCalculator {

public:
  IDFCalculator(DominatorTreeBase<BasicBlock> &DT) : DT(DT), useLiveIn(false) {}

  /// \brief Give the IDF calculator the set of blocks in which the value is
  /// defined.  This is equivalent to the set of starting blocks it should be
  /// calculating the IDF for (though later gets pruned based on liveness).
  ///
  /// Note: This set *must* live for the entire lifetime of the IDF calculator.
  void setDefiningBlocks(const SmallPtrSetImpl<BasicBlock *> &Blocks) {
    DefBlocks = &Blocks;
  }

  /// \brief Give the IDF calculator the set of blocks in which the value is
  /// live on entry to the block.   This is used to prune the IDF calculation to
  /// not include blocks where any phi insertion would be dead.
  ///
  /// Note: This set *must* live for the entire lifetime of the IDF calculator.

  void setLiveInBlocks(const SmallPtrSetImpl<BasicBlock *> &Blocks) {
    LiveInBlocks = &Blocks;
    useLiveIn = true;
  }

  /// \brief Reset the live-in block set to be empty, and tell the IDF
  /// calculator to not use liveness anymore.
  void resetLiveInBlocks() {
    LiveInBlocks = nullptr;
    useLiveIn = false;
  }

  /// \brief Calculate iterated dominance frontiers
  ///
  /// This uses the linear-time phi algorithm based on DJ-graphs mentioned in
  /// the file-level comment.  It performs DF->IDF pruning using the live-in
  /// set, to avoid computing the IDF for blocks where an inserted PHI node
  /// would be dead.
  void calculate(SmallVectorImpl<BasicBlock *> &IDFBlocks);

private:
  DominatorTreeBase<BasicBlock> &DT;
  bool useLiveIn;
  DenseMap<DomTreeNode *, unsigned> DomLevels;
  const SmallPtrSetImpl<BasicBlock *> *LiveInBlocks;
  const SmallPtrSetImpl<BasicBlock *> *DefBlocks;
  SmallVector<BasicBlock *, 32> PHIBlocks;
};
}
#endif